In a typical optical communication system, an optical transmitter generates an optical beam and modulates the beam with an electrical signal representative of the information to be transmitted by the communication system. Typically, an optical fiber propagates the modulated optical signal to a receiver that demodulates the optical beam to recover the electrical signal. The low loss, light weight, small size, flexibility and high intrinsic bandwidth of optical fiber make optical communication systems highly desirable for the communication of both of digital and analog signals.
There are many current and potential applications for optical communication systems, including cable TV (CATV) systems and telephone and other cross-country or cross-continent communication systems. One important application includes microwave or RF systems, typically used by the military, such as phased-array antenna systems, airborne radar warning-receiver direction-finding antenna systems, bi-static radar antenna systems, and many shipboard antenna systems. In most of these systems, a downconverter/upconverter is located in close proximity to the antenna so as to avoid the high losses associated with transmitting the microwave signals over inefficient metallic cables to the receiver or transmitter. The frequency converter operates in the typically harsh environment of the antenna, which increases the size and cost of the "front end" packaging of the downconverters/upconverters, and may limit placement of the antenna. Also, downconversion typically requires that a local-oscillator reference signal be distributed to the downconverter, again in the harsh environment of the antenna.
Accordingly, systems in which an RF or microwave signal is received or transmitted can benefit from direct microwave transport of the signal via an optical communication system between the antenna and the receiver/transmitter. Benefits can include removal of the frequency converter electronics from the "front end," a corresponding reduction in the size and complexity of the front end packaging, and improvements in overall system reliability, as fewer components are located in the harsh front end environment. The overall performance of the system can also be enhanced, as locating the frequency converter electronics at the front end typically limits the dynamic range of the system.
Unfortunately, the limitations of available optical transmitters can restrict the use of optical communication systems in RF, microwave and other systems. Available optical transmitters typically include a plurality of discrete components such as a laser, an external optical modulator and one or more control circuit modules interconnected by polarization-maintaining (PM) optical fiber. Currently, the optical transmitter is assembled from components that are housed in separate packages, namely, a standard DFB laser diode package and a modulator package, with possibly an optical tap coupler and wavelength reference in two other packages. Significant coupling losses are incurred at the laser-fiber and modulator-fiber interfaces, because lasers and modulators support elliptic modes while fiber medium supports a circular mode. The use of polarization maintaining fiber, such as on the fiber pigtails on the laser and modulator, adds cost because the fiber must be precisely rotated.
Other commercially-available optical transmitters include a laser assembly fixedly coupled to an optical modulator, both of which are rigidly mounted to a support bed. The optical components are fixedly coupled in an attempt to ensure precise alignment thereof, so as to reduce the power loss that can otherwise result from misaligned optics. Alignment of the optical components of these transmitters is difficult and time-consuming, increasing the costs of manufacturing.
Typically, these optical transmitters are sensitive to thermal changes as a result of the different coefficients of thermal expansion for the optical components. As the ambient temperature of the transmitter increases or decreases, unequal thermal expansion of the components creates stresses on the components and can alter their optical characteristics. Optical alignment of the optical components can also be affected. Because the optical beam emitted from the laser diode is typically directly focused to the modulator, misalignment is particularly detrimental, greatly reducing the output power of the transmitter as a result of the misalignment. Some prior art devices, such as those marketed by the G.E.C. Marconi company, are comprised of discrete components and include a thermocooler to help maintain temperature stability. However, these devices are not free from the aforementioned problems.
Furthermore, the optical components of existing optical transmitters are typically not readily replaceable or interchangeable. If a component has failed or it is desired to change the wavelength of the optical beam, the appropriate component cannot be easily removed or replaced without damage to the transmitter.
Additionally, a fiber-optic transmitter including these discrete components is relatively bulky and complicated. For example, currently available fiber optic transmitters produced for cable television (CATV) applications occupy a 19-inch rack drawer chassis, 3 inches or more high, housing power supplies, control circuits, laser, modulator, and amplifiers. As can be appreciated, such a transmitter may not be suitable for implementation at the front end of an antenna system.
The above concerns are especially relevant in Dense Wavelength Division Multiplexed (DWDM) systems, wherein multiple optical beams, each of a different wavelength and representing a distinct channel for the transmission of information, are multiplexed to propagate along a single fiber, thereby increasing the information carrying capacity of the fiber. Each channel typically requires its own optical transmitter, making the size and other limiting considerations of available optical transmitters, as discussed above, even more critical.
Accordingly, it is a principal object of the invention to address one or more of the foregoing disadvantages and deficiencies of the prior art.